The present invention relates to a transmission type liquid crystal display device.
Conventionally, in accordance with improvement in resolution of the transmission type liquid crystal display device, there have been proposed a variety of structures for effectively shading light without impairing an opening ratio of the liquid crystal display device in order to prevent a reduction in contrast due to malfunction ascribed to a photoelectric current and leak of light from the semiconductor layer of a thin film transistor that serves as a switching element.
As a typical transmission type liquid crystal display device, there is the one shown in FIG. 8 (Japanese Patent Laid-Open Publication No. HEI 9-43639). As shown in FIG. 8, this transmission type liquid crystal display device is constructed of a transparent drive substrate 101 and an opposite substrate 121 that are bonded to each other with interposition of a specified gap, and liquid crystals 120 are held in this gap.
The opposite substrate 121 is located on thie incident side of the liquid crystal display device, and an opposite electrode 122 is provided on the drive substrate 101 side. The drive substrate 101 is located on the radiating side and includes a group of pixels each constructed of a pixel electrode 115 and a switching element 103 on the drive substrate 101 and a black matrix for shading an unopened portion of individual pixels from light incident on the incident side. The black matrix is constructed of two layers of a mask light shading pattern 112 and a pad light shading pattern 113, which are patterned to overlap each other to complemertarily shade incident light. For example, an upper layer is made of titanium (Ti), while the lower layer is made of aluminum (Al).
The drive substrate 101 is constructed of an upper layer, an intermediate layer and a lower layer. The upper layer includes the pixel electrode 115. The lower layer includes the switching element 103 for driving each pixel electrode 115, a gate wiring (not shown) for scanning the rows of the switching elements 103 and a signal wiring (not shown) for supplying a specified image signal to the switching elements 103 of each column.
The switching element 103 is constructed of a thin film transistor (referred to as TFT hereinafter) that has a semiconductor thin film 102 made of polycrystal silicon or the like as an active layer. A gate electrode 105 is formed on the transistor via a gate insulating film 104. The gate electrode 105 leads to the gate wiring. The TFT that serves as the switching element 103 is provided with a source region and a drain region on both sides of the gate electrode 105. One lead electrode 109 is connected to the source region of the semiconductor thin film 102, and the lead electrode 109 leads to a signal wiring (not shown) The other lead electrode 110 is connected to the drain region of the semiconductor thin film 102.
Further, the semiconductor thin film 102 is also provided with an auxiliary capacitance. This auxiliary capacitance includes the semiconductor thin film 102 that serves as one electrode and an auxiliary capacitance wiring 107 that serves as the other electrode via an insulating film 106. The gate electrode 105, the gate wiring and the auxiliary capacitance wiring 107 are constructed of a same layer and insulated from the lead electrodes 109, 110 by a first interlayer insulating film 108.
Then, bisected light shading patterns 112, 113 exist in the intermediate layer. One light shading pattern 112 is continuously patterned along the direction of row of the pixels and at least partially shades light to the switching element 103 while being vertically held by a second interlayer insulating film 111 and a flattening film 114 so as to be insulated from the lower layer and the upper layer. The other light shading pattern 113 is discretely patterned to be interposed between the pixel electrode 115 and the lead electrode 110, so that the light shading pattern 113 is for good electrical connection between both the electrodes as well as light shading. The lead electrode 110 is formed of a layer identical to that of the signal wiring and electrically connected to the drain region of the semiconductor thin film 102.
In the above-mentioned transmission type liquid crystal display device, no light shading is needed if a metal or silicide is used for the gate wiring, gate electrode 105, auxiliary capacitance wiring 107, signal wiring and lead electrodes 109, 110. However, the semiconductor layer of the TFT formed of the semiconductor thin film 102 is required to be completely shaded from light due to the light transmission of the semiconductor layer. Therefore, in order to completely shade the region to be shaded from light in the lower layer by the light shading patterns 112, 113, the light shading patterns are required to be formed jutting out of the region (semiconductor layer of TFT) to be shaded from light taking variations in alignment and line width through the patterning process into consideration. The opening ratio is reduced by an area of the jut region, and this causes a problem that brightness of the liquid crystal display device is reduced.
Furthermore, due to the necessity of the film formation of the light shading patterns, the process is prolonged and the structure becomes complicated, disadvantageously leading to a factor of yield reduction and cost increase. Specifically, there are needed the processes of forming the interlayer insulating films, light shading patterns and contact holes.
FIG. 9 shows a sectional view of the essential part of another conventional transmission type liquid crystal display device, in which a lower layer insulating film 203, a TFT 204, a first interlayer insulating film 205, a gate electrode 206, an auxiliary capacitance wiring 207, a signal wiring 208, a second interlayer insulating film 209 and a pixel electrode 210 are successively formed on a transparent substrate 201. With regard to the above transmission type liquid crystal display device, light comes from the above and goes out downward in FIG. 9. A reflected light R1 from an optical system on the radiating side may be incident on the TFT 204, and a reflected light R2 caused by reflection of a transmitted light on the lower surface of the transparent substrate 201 may be incident on the TFT 204. This leads to a problem that the reflected lights R1 and R2 are incident on the TFT to cause an optical leak current when the TFT is turned off in accordance with the dimensional reduction of pixels in company with the improvement in resolution.
An object of the present invention is to provide a transmission type liquid crystal display device that can cope with an improvement in resolution by effectively shading light without impairing a opening ratio and by shading a rear surface reflection light, the device being able to be fabricated with a simple construction and high yield at low cost through a short process.
In order to achieve the above-mentioned object, the present invention provides a transmission type liquid crystal display device including on a transparent substrate a gate wiring; a signal wiring perpendicular to the gate wiring, an auxiliary capacitance wiring that is generally parallel to the gate wiring and perpendicular to the signal wiring, a thin film transistor having either one of a source region and a drain region connected to the signal wiring, and a pixel electrode to which the other one of the source region and the drain region of the thin film transistor is connected via a lead electrode, wherein the signal wiring, the gate wiring, the auxiliary capacitance wiring and the lead electrode are made of a light shading material, a semiconductor thin film is formed for each pixel electrode below the signal wiring, the gate wiring, the auxiliary capacitance wiring and the lead electrode via an insulating film, a region that belongs to the semiconductor thin film and is located below the signal wiring and below the gate wiring is made to serve as a channel region of the thin film transistor, regions that belong to the semiconductor thin film and are located on both sides of the channel region below the signal wiring are made to serve as a source region and a drain region of the thin film transistor respectively, and a region that belongs to the semiconductor thin film and is located below the auxiliary capacitance wiring is made to serve as an auxiliary capacitance electrode region.
According to the transmission type liquid crystal display device of the present invention, the gate wiring, the signal wiring, the auxiliary capacitance wiring and the lead electrode are formed of a light shading material. Therefore, the channel region of the TFT, which is located below the signal wiring and below the gate wiring, is shaded from incident light. The source region and the drain region of the TFT, which are located on both sides of the channel region below the signal wiring, are also shaded from incident light. Accordingly, the TFT is entirely shaded from incident light. Since the gate wiring, the signal wiring, the auxiliary capacitance wiring and the lead electrode are made of the light shading material, it is not necessary to form a light shading film for shading the TFT. This allows the a transmission type liquid crystal display device to be fabricated at low cost and with high yield. Light shading of the TFT is effectively achieved with a simple construction without impairing the opening ratio of a liquid crystal display.
In an embodiment of the invention, a jut region of the semiconductor thin film in which juts from the signal wiring, the gate wiring, the auxiliary capacitance wiring and the lead electrode has an areal ratio of 0.1 or less with respect to an area of an opening through which light is transmitted.
According to the transmission type liquid crystal display device of the above embodiment, it is preferred that the semiconductor thin film be completely covered. However, even if the semiconductor thin film juts over the opening through which light is transmitted, the jut region of the semiconductor thin film cannot be visually distinguished by human beings so long as the jut region has an area of not greater than 10% of the total area of the opening.
In an embodiment of the invention, the transmission type liquid crystal display device further comprises a lower layer light shading film formed below the semiconductor thin film and on the transparent substrate so as to cover a region that includes at least the channel region of the thin film transistor of the semiconductor thin film.
According to the transmission type liquid crystal display device of the above embodiment, the rear surface reflection light to the TFT is obstructed by forming the lower layer light shading film below at least the channel region, the source region and the drain region of the TFT in the semiconductor thin film via the insulating film.
In an embodiment of the invention, the lower layer light shading film is formed on the transparent substrate so as to cover a region located between the gate wiring and the auxiliary capacitance wiring.
According to the transmission type liquid crystal display device of the above embodiment, the region between the gate wiring and the auxiliary capacitance wiring is shaded by the lower layer light shading film.
In an embodiment of the invention, the transmission type liquid crystal display device further comprises a first contact hole for connecting either one of the source region and the drain region of the semiconductor thin film to the signal wiring, a second contact hole for connecting the other one of the source region and the drain region of the semiconductor thin film to the lead electrode, and a third contact hole for connecting the lead electrode to the pixel electrode,
the signal wiring being electrically connected to the pixel electrode via the first contact hole, the source region, the channel region, the drain region, and the auxiliary capacitance electrode region of the semiconductor thin film, the second contact hole, the lead electrode and the third contact hole.
According to the transmission type liquid crystal display device of the above embodiment, if the above-mentioned TFT is turned on, then the potential of the signal wiring is applied to the pixel electrode via the first contact hole, the source region, the channel region, the drain region and the auxiliary capacitance electrode region of the semiconductor thin film, the second contact hole, the lead electrode and the third contact hole. Then, the potential of the pixel electrode is retained in the capacitance formed by holding liquid crystals between the pixel electrode and the opposite electrode (located on the opposite substrate side), and the potential of the pixel electrode is retained in the auxiliary capacitance formed by holding the insulating film that serves as a dielectric film between the auxiliary capacitance wiring and the auxiliary capacitance electrode region of the semiconductor thin film.
In an embodiment of the invention, the gate electrode and the auxiliary capacitance wiring are made of a same material.
According to the transmission type liquid crystal display device of the above embodiment, the gate wiring and the auxiliary capacitance wiring of the same material is formed, so there can be obtained a simple construction in which the gate wiring and the auxiliary capacitance wiring can be formed in a same layer through a same process.
In an embodiment of the invention, the signal wiring and the lead electrode are made of a same material.
According to the transmission type liquid crystal display device of the above embodiment, the signal wiring and the lead electrode of the same material is formed, so there can be obtained a simple construction in which the signal wiring and the lead electrode can be formed in a same layer through a same process.
In an embodiment of the invention, the lead electrode is a thin film whose a principal component is Al, and a film layer which contains at least one substance selected from among Ir, Ru, Cr, Co, Ta, Ti, W, Mo, TiW alloy, WN, TiN and a silicide of Ir, Cr, Co, Ta, Ti, W and Mo is laminated on the lead electrode
According to the transmission type liquid crystal display device of the above embodiment, the film layer containing at least one substance selected from among Ir, Ru, Cr, Co, Ta, Ti, W, Mo, TiW alloy, WN, TiN and the silicide of Ir, Cr, Co, Ta, Ti, W and Mo is laminated on the lead electrode whose a principal component is Al. Therefore, the pixel electrode which is made of a transparent oxide such as ITO (indium-tin-oxide) does not directly contacts the Al thin film of the lead electrode, and thereby Al oxide is not formed at an interface between the lead electrode and the pixel electrode, allowing good electrical connection between the lead electrode and the pixel electrode to be obtained.